1. Technical Field
This invention relates to an optical recording medium provided with a substrate having an uneven pattern formed on at least one principal surface and a method of manufacturing such an optical recording medium.
2. Background Art
Optical recording media are recording media used with respect to recording/reproducing apparatus and adapted so that recording and/or reproduction (hereinafter referred to as recording/reproduction) of various information, e.g., audio (sound) information, picture information or character information, etc. is carried out by such recording/reproduction apparatus (unit). Optical recording media generally have a disc shape. There are various kinds of optical discs, including, for example, an optical disc in which recording signals are written in advance by physical uneven shapes called pits, a phase change type optical disc in which recording signals are written by making use of the phase change of a signal recording layer, and magneto-optical discs in which recording/reproduction of recording signals is carried out by making use of the magneto- optical effect of a signal recording layer, etc.
At the optical recording medium, there are provided recording portions for recording signals called recording tracks in the circumferential direction on the principal surface thereof. Further, the recording/reproducing unit irradiates light toward these recording tracks to carry out recording/reproduction of recording signals. At the optical recording medium, e.g., land grooves which are physical uneven shapes corresponding to recording tracks and/or fine uneven patterns such as pits, etc. written in advance are formed on the signal recording surface. Such fine uneven patterns are ordinarily formed by transfer by means of a stamper onto the side serving as a signal recording surface of this disc substrate when the disc substrate is injection-molded by resin material. Namely, at the time of forming the substrate having an uneven pattern, the stamper having a reversed uneven pattern in which an uneven pattern is reversed is first formed to carry out injection molding using such a stamper.
Meanwhile, in the above-described injection molding process for the disc substrate, by influences such as pressure change and temperature change at the time of injection-filling molten (fused) resin material and/or friction between resin material and metal mold, etc., there takes place stress such as molecular orientation distortion or thermal distortion, etc. in this resin material. Further, in this injection molding process, in the process until molten resin material is cooled and hardened within the metal mold, stress produced in the resin material is relaxed, but a partial pressure which has remained without being relaxed is left as residual stress within the completed disc substrate. In such optical recording mediums, when internal stress remains within the disc substrate, there may take place deformation such as curvature or undulation, etc. and/or uneven double refraction distribution, etc.
Further, since the disc substrate is formed by resin material, it is difficult to avoid contraction deformation particularly in the cooling process of the injection molding process. In a more practical sense, in the case where contraction at the outer circumferential portion of the disc substrate is greater than that at the inner circumferential portion thereof, the disc substrate is deformed so as to take a propeller shape. In addition, in the case where contraction at the outer circumferential portion is greater than that at the inner circumferential portion, the disc substrate is deformed so as to take a bowl shape.
For this reason, in the conventional injection molding process, in order to suppress, to the utmost, such contraction deformation or deformation of disc substrate by the above- described residual internal stress, complicated labors are required such as weakening the clamping pressure of the metal mold, lowering the injection speed of resin material and/or weakening the injection pressure of resin material. In addition, in the conventional injection molding process, such deformation is produced, whereby concave (hollow) portion or swollen portion takes place at the outer circumferential portion of the disc substrate, or it becomes difficult to transfer, with good accuracy, reversed uneven patterns of stamper.
For this reason, in the conventional injection molding process, resin material having low molecular weight is used to enhance fluidity of resin material or to increase temperature at the time of injection of this resin material and temperature of the metal mold to thereby realize reduction of double refraction of the disc substrate and to ensure transfer characteristic from the stamper. However, in the conventional injection molding process, there is the problem that the molecular weight of resin material is reduced so that mechanical characteristic such as rigidity, etc. of the disc substrate is degraded. In addition, in the conventional injection molding process, there is the problem that metal mold temperature is increased so that thermal deformation takes place at the time of taking out the disc substrate, or the molding cycle is elongated.
Meanwhile, in this optical recording medium, with the increase of the quantity of data to be handled in computer equipment, etc., realization of large capacity is required. In order to realize large capacity, it is necessary to increase information quantity per unit area, i.e., promote realization of high density recording. In the optical recording media, it is possible to carry out, with high density, recording/reproduction of signals by lasers of short wavelength and optical systems using object lens (objective) having a high N.A. (Numerical Aperture) at the time of recording/reproduction.
However, in the case where such an optical disc is used, focal depth of the object lens becomes small, and spot diameter of the laser becomes small. As a result, the position of the object lens used for recording/reproduction is caused to be in the vicinity of the surface from the bottom surface of the optical recording medium. Accordingly, the allowed range of curvature or undulation, etc. of the substrate becomes small. This is apparent from the relational expression between N.A. of the object lens and substrate thickness and the relationship between wave length and N.A.
f=D/2NA greater than WD greater than 
f: Focal distance of lens
D: Effective diameter of object lens
N.A.: Numerical Aperture of object lens
WD: Actuation distance of the object lens
xe2x80x83Focal depth xe2x88x9dxc3xa/(NA)2
SKEW allowed value xe2x88x9dxc3xa/(NA)3
Thickness unevenness allowed value xe2x88x9dxc3xa/(NA)4
Strength of disc xe2x88x9d(thickness)3
When the relationship between NA and thickness of optically transparent substrate is determined from the above-mentioned relational expressions, there are provided the following relationships.
In the case where NA=0.5, substrate thickness 1.2 mm.
In the case where NA=0.6, substrate thickness 0.6 mm.
In the case where NA=0.75, substrate thickness 0.3 mm.
In the case where NA=0.85, substrate thickness 0.1 mm.
As stated above, in the optical recording media, it is necessary to thin the substrate for high density recording. However, since strength of the substrate has the relationship proportional to the third power of thickness, when the substrate is caused to be of thin thickness the strength of the substrate is lowered substantially. In addition at the time of manufacturing the thin substrate, the substrate is molded while being solidified resulting from a temperature difference between metal mold and resin. Thus, orientation distortion becomes more conspicuous. For example, in the case where thickness of the substrate is 0.4 mm or less, degradation of transfer characteristic, unevenness of double refraction and occurrence of curvature become conspicuous along with orientation distortion.
For this reason, in the optical recording medium, as material for forming substrate, it is desirable to use material excellent in bending and tensile strength and having high rigidity. As material stated above, there is mentioned compound mold material in which filler or short fiber is mixed into ordinary resin material. However, with such compound mold material, the surface has a pear-skin or rough property by filler or short fiber. For this reason, in the case where the compound mold material is used, there is the problem that it is impossible to precisely transfer fine uneven patterns in order to manufacture substrate suitable for optical recording media which cope with high density recording.
Disclosure of the invention
An object of this invention is to provide an optical recording medium in which there is no deformation such as curvature or concave portion of the disc substrate, having sufficient rotation performance, and which can sufficiently cope with realization of high recording density and a method of manufacturing such an optical recording medium.
The optical recording medium according to this invention is directed to an optical recording medium comprising a substrate having an uneven pattern formed on at least one principal surface and adapted so that light is irradiated onto the uneven pattern and recording and/or reproduction of recording signals are carried out by the light, the substrate including an intermediate layer including a rigidity rendering material and a surface layer formed on at least one principal surface of the intermediate layer and having the uneven pattern formed thereon.
In the optical recording medium according to this invention thus constituted, rigidity of the substrate is enhanced by the intermediate layer including the rigidity rendering material, and the uneven pattern is formed on the surface layer formed on the intermediate layer. For this reason, in the optical recording medium it is not possible for the rigidity rendering material to face the surface where the uneven pattern is formed. Thus, the optical recording medium exhibits excellent signal characteristics and has excellent rigidity.
In addition, the method of manufacturing an optical recording medium according to this invention is directed to a method of manufacturing an optical recording medium comprising a substrate having an uneven pattern formed on at least one principal surface, and adapted so that light is irradiated onto the uneven pattern and recording and/or reproduction of recording signals are carried out by the light, wherein a surface layer is disposed in a manner to oppose one principal surface of a stamper where reversed uneven pattern is formed on the one principal surface and an intermediate layer including an rigidity rendering material is stacked on the surface layer to pressure-fit the stamper, the surface layer and the intermediate layer to thereby integrate the surface layer and the intermediate layer and to transfer the reversed uneven pattern onto the surface layer.
In the method of manufacturing the optical recording medium according to this invention thus featured, there is employed such an approach to transfer a reversed uneven pattern formed on the stamper onto the surface layer to integrate the surface layer and the intermediate layer including the rigidity rendering material to thereby form substrate. For this reason, in accordance with this technique, it is possible to manufacture a substrate having excellent rigidity and to transfer a reversed uneven pattern with high accuracy.